Shape Evolution of Droplets Growing on Linear Microgrooves

Abstract

Anisotropic spreading of liquids and elongated droplet shapes are often encountered on surfaces decorated with a periodic micropattern of linear surface topographies. Numerical calculations and wetting experiments show that the shape evolution of droplets that are slowly growing on a surface with parallel grooves can be grouped into two distinct morphological regimes. In the first regime, the liquid of the growing droplet spreads only into the direction parallel to the grooves. In the second regime, the three-phase contact line advances also perpendicular to the grooves, whereas the growing droplets approach a scale-invariant shape. Here, we demonstrate that shapes of droplets in contact with a large number of linear grooves are identical to the shapes of droplets confined to a plane chemical stripe, where this mapping of shapes is solely based on the knowledge of the cross section of the linear grooves and the material contact angle. The spectrum of interfacial shapes on the chemical stripe can be exploited to predict the particular growth mode and the asymptotic value of the base eccentricity in the limit of droplets covering a large number of grooves. The proposed model shows an excellent agreement with experimentally observed base eccentricities for droplets on grooves of various cross sections. The universality of the model is underlined by the accurate match with available literature data for droplet eccentricities on parallel chemical stripes.

Directional Liquid Wicking in Regular Arrays of Triangular Posts

“…These surfaces could be tuned to show uni-, bi-, and tridirectional spreading of partially wetting liquid. Liquids deposited on chemically homogeneous solid surfaces decorated with micropatterns of parallel grooves may display anisotropic droplet shapes or even spread solely parallel to the grooves. − …”

Polymer Spreading on Unidirectionally Nanotextured Substrates Using Molecular Dynamics

“…Thus, in agreement with Kubiak and Mathia 9 and Fischer et al, 10 both peaks and grooves modify spreading, and we observe the pinning-depinning-repinning spreading mechanism previously described. 10,11,14,19,20 We also observe that the sharp θ = 60°substrate grooves and peaks inhibit perpendicular spreading longer than the sinusoidal wave grooves and peaks.…”

“…Alternatively, the liquid may only be slowed by texture peaks, causing the contact line to advance in a stick-slip manner, pinning-depinning-repinning on subsequent texture peaks. 10,11,14,19,20 Conversely, others propose that anisotropic spreading is caused by texture grooves that accelerate spreading along the direction parallel to the texture, i.e., along the grooves. 9,10,15−18 Fischer et al 10 this accelerated spreading in part to capillary action inside the grooves, based on experiments with sinusoidal textures (height of 8 or 16 μm).…”

Biaxially Morphing Droplet Shape by an Active Surface

“…After depositing a droplet (2 µL) on the surface (Figure S1, Supporting Information), anisotropic wetting is realized (Figure 1c) for plain surface (Movie S1, Supporting Information), patterned surface without wrinkles (Movie S2, Supporting Information) and patterns surface with wrinkles (Movie S3, Supporting Information) under compression, where an elongated droplet with high aspect ratio oval shape is achieved by spreading along winkle patterns. [ 45–47 ] Directions perpendicular (⊥) and parallel (||) to the wrinkle/groove direction are defined to describe surface topography and droplet shape (wetting anisotropy). Due to the pinning of contact line (Figure 1d), the contact angle (CA) perpendicular to groove appears larger than the parallel one.…”